Great you are looking into this. Someone really ought to look at the performance stuff, since there is a lot of low hanging fruit there for SMW :)
I think the fruits are sweet :)
Although optimizing that would be nice, I guess the deal performance issues people are having are the many SMW related read queries that happen at page render, or even at page view. One thing that falls into the later category is the Semantic Forms check to see if a page is in a certain namespace to see if it should have a certain "edit with form" tab, which consists out of several queries. Stuff like that could be cached using whatever cache is available to MediaWiki, so that all the needed info can be obtained using a single request to this cache. (Such caches can be stuff such as memcached, but if those are not available, it falls back to the db, still reducing the amount of queries to one.)
I think we can implement caching in Semantic Forms and other places as mentioned here
Also we might use APC for caching PHP code, as described here
Both Memcache and APC can run in parallel so we might also consider using both
Thanks for this link.